Typically a ceramic or glass lined hot water tank includes a cylindrical side wall with a cylindrical top having a concave interior surface to define a "plus" head for the tank. The bottom of the tank has a convex interior surface to define a "minus" bottom for the tank. This arrangement has been manufactured for a considerable number of years where the technique is normally to weld the top to the cylindrical side wall rolled from a flat rectangular blank and to coat the interior of the bottom open ended tank. A precoated bottom having the convex interior surface is then welded to the base of the tank.
Although such construction is acceptable for hot water tanks, it has been found that tank failure is a problem in the industry normally due to the hot water at the top of the tank which increases the corrosion at the joint between the tank top and the side wall. Such failure is normally caused by interruption in the glazing or coating applied to this area during the coating process. An additional drawback of this construction is that the "minus" bottom for the tank requires a material thickness considerably thicker than the side wall of the tank to ensure that under pressure the tank bottom, at least in its central area, does not flex which could crack or in one way or another disrupt the coating. An additional drawback with the "minus" head construction is that it subtracts considerably from the assembled tank volume, hence the overall height of the hot water tank must be greater. The hot water at the top of the tank increases any corrosive reaction because released gases, which includes oxygen, are free to chemically react with and corrode any bare metal exposed by poor glazing or coatings. It is also appreciated that the speed of chemical reaction doubles with every 10.degree. C. increase in temperature. In addition the abutting nature of the fit of the tank top with the tank side is not necessarily perfect because of nicks or other imperfections in the outer periphery of the cylindrical portion of the tank side wall and the top. Hence it is quite possible, even with the greatest of care in applying the glazing material that some uncoated areas exist at the joint between the tank top and the tank side wall.
To increase the tank volume, it would be beneficial to provide not only a "plus" head portion for the hot water tank but also a "plus" bottom portion. This would increase capacity of the tank for a given diameter with possible reduction in tank height depending on the shape of the "plus" head. However with the existing technology, there is no known approach to connect the "plus" bottom to the tank after the tank with welded "plus" top has been coated with ceramic material. If a "plus" bottom end for the tank as precoated were welded to the tank side wall in the manner in which the top is welded, the coating would be severely degraded or damaged due to the heat of welding. Such degradation would result in uncoated portions of steel which would be corroded by the hot water.
Considering prior art tank constructions which provide "plus" heads and bottoms for cylindrical tank construction, U.S. Pat. No. 3,199,711 discloses a fire extinguishing tank which may contain pressurized materials. The tank is constructed of materials which are not corroded by the contents hence no protective or corrosion inhibiting coating is applied to the tank interior. For example in containing fire extinguishing liquids, the tank may be formed of copper or the like where the components are soldered together. A similar system is provided in U.S. Pat. No. 3,952,904 wherein again the material of construction is not corroded by its contents. The plastic bottom of concave interior shape is sonic welded to the plastic side walls of the barrier where a seal is provided at the connection to prevent leakage. No protective coating is required in the system nor is any welding used in making the connection where the heat of the weld could in any way affect an interior protective coating.
Other types of containers having "plus" end portions on cylindrical tank side walls are disclosed in U.S. Pat. Nos. 3,098,577 and 3,132,618. Neither patent contemplates interior coating of the tank surfaces to inhibit corrosion caused by its contents. The considerations given above to the prior form of hot water tank construction is not solved by the tank construction disclosed in these patents.